The glycosaminoglycans such as heparin, heparan sulfate, dermatan sulfate, chondroitin sulfate and hyaluronic acid are biopolymers that are industrially extracted from various animal organs.
In particular, heparin, mainly obtained by extraction from pig intestinal mucosa or from bovine lung, is a polydispersed copolymer with a molecular weight distribution from approximately 3,000 to approximately 30,000 D consisting of a mixture of chains essentially consisting of a uronic acid (glucuronic acid or iduronic acid) and of an amino sugar (glucosamine) linked by α-1→4 or β-1→4 bonds. In heparin, the uronic unit can be O-sulfated in position 2 and the glucosamine unit is N-acetylated or N-sulfated, 6-O-sulfated, and 3-O-sulfated in approximately 0.5% of the present glucosamine units.
The properties and natural biosynthesis of heparin in mammals have been described by Lindahl et al., 1986 in Lane, D. and Lindahl, U. (Editors) “Heparin. Chemical and Biological Properties; Clinical Applications”, Edward Arnold, London, Pages 159-190, by Lindahl, U, Feingold D. S. and Rodén L, 1986 TIBS, 11, 221-225 and by Conrad H. E. “Heparin Binding Proteins”, Chapter 2: Structure of Heparinoids. Academic Press, 1998. The biosynthesis of heparin occurs starting with its precursor N-acetylheparosan consisting of a chain mixture consisting of the repetitive disaccharide unit glucuronyl-β-1→4-N-acetylglucosamine. Said precursor undergoes enzymatic modifications which partially hydrolyze the N-acetyl group, substituting it with an SO3− group, epimerize the carboxyl in position 5 of a part of the glucuronic units converting them into iduronic units and introduce O-sulfate groups to get a product which, once extracted industrially, has approximately double the number of sulfate groups in respect of the carboxyl ones per disaccharide unit. These enzymatic modifications lead, among others, to the formation of the pentasaccharide region binding to antithrombin III (ATIII), called active pentasaccharide, which is the structure necessary for the high affinity binding of heparin to the ATIII and fundamental for anticoagulant and antithrombotic activity of the heparin itself. This pentasaccharide, present inside only some of the chains which form heparin, contains a glucosamine unit sulfated in position 3 and a glucuronic acid spaced out between disaccharides containing iduronic acids.
In nature, the formation of the active pentasaccharide is made possible by the epimerization reaction of the carboxyl of a part of the glucuronic units into iduronic units by the glucuronyl-C5-epimerase (C5-epimerization) and by suitable sulfation which also leads to the introduction of a sulfate group onto the hydroxyl in position 3 of the glucosamine. More particularly, in nature the formation of the active pentasaccharide is made possible by the fact that the C5-epimerization occurs in clusters, i.e. on portions of chains, and extensively, which results in a product that contains more iduronic units than glucuronic ones. Commercial heparin, in fact, contains approximately 70% of iduronic units and 30% of glucuronic units.
Alongside the main anticoagulant and antithrombotic activities, heparin also exercises antilipemic, antiproliferative, antiviral, antitumor and antimetastatic activities, but its use as a drug is hindered by the side effects due to the anticoagulant action which can cause bleeding.